Device and method for use in quantum crytography

ABSTRACT

The present invention relates to a quantum cryptography system for the secure key generation, especially with signal sources and analysis channels. The signal sources are arranged spatially separated in such a manner that the wave fronts of light signals emitted by them superimpose partially at the input of the quantum channel. The analysis channels are arranged in such a manner that the wave front of the light signals coming from the quantum channel are split up spatially and at least two of the parts are analyzed in a quantum mechanical state.

BACKGROUND OF THE INVENTION

Quantum cryptography facilitates a quantifiable secure communication. Bythe transmission of quantum particles, particularly photons, anarbitrary, secure key can be generated. This key can then be used forthe enciphering of corresponding methods (e.g., one-time-pad, DES).Possible eavesdropping attempts change the quantum particles in a waythat errors in the generated key show the attack. This is inconsiderable contrast to the conventional system in which the securityof the transmission is based, e.g., on the faith in couriers or on(unproved) assumptions of the technological abilities of theeavesdropping person.

Quantum cryptography was theoretically suggested for the first time in1984 and experimentally realized in 1991. Several theoretical andexperimental publications and patents led to a rapid development in thisfield. At present, the research in the field of quantum cryptographyconcentrates over all on the technical implementation of firstprototypes. Here great attention is paid to the miniaturization of thesystems used as well as on a high stability and economic efficiency ofthe transmitter and receiver optics.

The prior art relevant for the present invention is documented, e.g.,by:

-   [1] U.S. Pat. No. 5,307,410 Interferometric quantum cryptographic    key distribution system CH. H. Bennett-   [2] U.S. Pat. No. 5,732,139 Quantum cryptographic system with    reduced data loss H-K. Lo, H. F. Chau-   [3] EP 0 776 558 Quantum cryptography P. D. Townsend-   [4] U.S. Pat. No. 5,243,649 Apparatus and method for quantum    mechanical encryption for the transmission of secure    communication J. D. Franson-   [5] EP 0 923 828 Quantum cryptography device and method N. Gisin, A.    Mueller, B. Perny, H. Zbinden, B. Huttner-   [6] EP 0 717 895 B1 Key distribution in a multiple access network    using quantum cryptography P. D. Townsend, D. W. Smith-   [7] U.S. Pat. No. 5,757,912 System and method for quantum    cryptography K. J. Blow-   [8] EP 0 722 640 B1 Cryptographic receiver J. G. Rarity, P. R.    Tapster-   [9] Towards practical quantum cryptography S. Chiangga, P. Zarda, T.    Jennewein, H. Weinfurther Appl. Phys. B. 69, 389 (1999)-   [10] Daylight quantum key distribution over 1.6 km W. T.    Buttler, R. J. Hughes, S. K. Lamoreaux, G. L. Morgan, J. E.    Nordholt, C. G. Peterson Phys. Rev. lett. 84, 5652 (2000)-   [11] Long distance entanglement based quantum key distribution G.    Ribordy, J. Brendel, J.-D. Gautier, N. Gisin, H. Zbinden Phys. Rev.    A 63, 012309 (2001)

A summarizing documentation can also be found in the article“Quantenkryptographie” by U. Gebranzig, W. Süβmuth, Jahrbuch desdeutschen Patentamts 1999.

The present invention relates to a system for the secure distribution ofcryptographic keys according to the method of the quantum cryptography.In the quantum cryptography, as known from U.S. Pat. No. 5,307,410 [1]and U.S. Pat. No. 5,732,139 [2], a cryptographic key is generated bytransmitting information carrying light signals between two or moreparticipants on the quantum channel, by measuring said signals and byexchanging information on the measured values via a conventionalcommunication channel. Possible eavesdropping attacks during thetransmission of the light signals can be detected according toconclusions of the quantum theory. The generated cryptographic key is ofgreat importance for the transmission of all kinds of information due toits high security.

From [3]-[5], apparatuses are known, which are suitable for the keydistribution in accordance with the principles of quantum cryptography.In particular, transmitter and receiver are described, which providerapid switches for changing quantum mechanical states of the lightsignals transmitted by a signal source and detected by an analysischannel. Here, disadvantages are the high costs and the high technicalefforts being necessary for the rapid operation of the switches.

According to the patents [6] and [7], the efforts can be reduced. Afirst simplification is described in EP 0 717 895 B1 [6]. Hereapparatuses for quantum cryptography are described, in whichnon-orthogonal quantum states are coupled by an optical switch into theoutput of the transmission unit. U.S. Pat. No. 5,757,912 [7] describes amethod in which 2 sources generate orthogonal light states. Thus, areduction in the efforts by the factor 2 for the phase modulation isachieved.

From documents [8]-[11], apparatuses are known in which no switches arenecessary due to the use of 2 or more signal sources or 2 or moreanalysis channels. Here, the light signals emitted by the signal sourcesare superimposed in the transmitter device by means of opticalcomponents, especially semipermeable mirrors, or the light signals aresplit in the receiver device by means of a semipermeable mirror. Here,the signal sources are controlled in such a way that only one of thesources generates at a time one single photon or a reduced light impulseat the output of the transmitter device. According to patent EP 0 722640 B1 [8], in the receiver the incoming photon is distributed at randomon the different analyzers by a beam splitter and is registered by oneof the detectors, thus forming the signal relevant for the keygeneration.

During the technical realization, it has proven especiallydisadvantageous that therefor the direction of the signal sources has tobe accurately adjusted and that necessary components, particularly thesemipermeable mirrors, change the quantum mechanical state in anundesired manner. For its correction, further optical components have tobe introduced and to be adjusted accurately. The high number of opticalcomponents and the high adjusting efforts cause an increased spacerequirement, a non-optimal signal/noise ration and a bad stability ofthe systems. It has also proven disadvantageous that especially for theuse of new, more efficient methods (U.S. Pat. No. 5,732,139 [2]), avariation of the splitting ration can only be achieved by exchanging andadjusting optical components anew.

The present invention is based on the object to provide an improvedapparatus and an improved method for the quantum cryptography.

This object is solved with the subject-matter of the claims.

SUMMARY OF THE INVENTION

It is an advantage of the present invention that existing apparatusescan be simplified with regard to the number and kind of the opticalcomponents and thus to facilitate a further miniaturization, an increaseof the flexibility as well as an improvement of the signal/noise ratiounder consideration of the stability and the economical efficiency.

According to the present invention, the signal sources are spatiallyarranged in such a manner that the wave fronts of the light signalsemitted by said sources partially superimpose each other at the input ofthe quantum channel and/or the analysis channels are spatially arrangedin such a manner that the wave fronts of the light signals coming fromthe quantum channel are spatially split up and at least 2 of the partsare analyzed in a quantum mechanical state.

In an advantageous embodiment of the present invention, mirrors andother components changing the wave fronts are used. Thus the size of thetransmitting and receiving optics can be reduced considerably, withoutdisturbing the quantum mechanical states of the light signals. Ascomponents changing the wave fronts, mirrors, prisms, glass plates,lenses and/or diffractive elements can be used.

By deferring the geometrical arrangement of a mirror or the analysischannels within the light cone of the receiver unit, the splitting ratioonto the different analysis channels can be adjusted and be optimizedfor the special case.

The arbitrary splitting at random onto several analysis channels isguaranteed by the light cone being spatially split up. In accordancewith the quantum mechanic, the detection of a single photon light signalis arbitrary and not determined in the different parts of the lightcone.

It is a special advantage of the present invention that the spatialsuperimposing or splitting of the light cone can be used flexibly forseveral kinds of quantum mechanical states. In particular, in anembodiment according to the present invention, the quantum mechanicalstate can be realized by the property of polarization of the lightsignals, or alternatively, by a phase difference of components having anoffset in time of the light signal or by a phase difference of frequencycomponents of the light signal.

In an advantageous version of the present invention, the analysischannels are formed in such a manner that the optical components can becommonly used for several channels without loosing the completefunctionality. This reduces the complexity, the costs and the size ofthe receiver optic.

The present invention is formed by the geometrical superimposition ofthe light beams generated by the signal sources at the output as well asthe geometrical splitting of the light beam in the receiver. Thecasualty necessary for the quantum cryptography in the receiver isguaranteed, contrary to the known systems, not by the casualty of thedetection in the output of the beam splitter but by the casualty of thedetection in geometrically different parts of the light beam.

The following features of the present invention are advantageous orpreferred:

-   -   The present invention comprises a cryptographic transmitter with        at least 2 signal sources, the transmitter in which at least 2        of the prevailing signal sources are non-orthogonal in the sense        of a quantum mechanical preparation and which is characterized        in that the light coming from the signal sources spatially        superimposes at the output of the transmitter and enters the        quantum channel.    -   The spatial superimposition can be achieved either solely by the        divergence of the beam of the signal sources or by means of        mirrors, lenses, prisms or refractive elements.    -   The present invention further comprises a cryptographic receiver        unit with at least 2 measuring units for quantum mechanical        properties, wherein at least 2 of the measuring units are        oriented non-orthogonally in the sense of a quantum mechanical        measuring, the receiver unit being characterized in that the        light beam coming from the quantum channel is split up        geometrically between the measuring units.    -   This is achieved either by the geometrical arrangement of the        measuring unit within the light beam or by the introduction of        mirrors, lenses, prisms or refractive elements in the light        beam.    -   The splitting ratio onto the different measuring units can be        varied by changing the position of the measuring unit or of the        introduced components (mirror, lens, prism, glass plate,        refractive element).    -   According to the quantum mechanic, it is undetermined and        arbitrary in which part of the light beam, i.e., in which of the        measuring units, a single photon coming from the quantum channel        is detected.    -   A quantum cryptography system can use either a transmitter        described above or a receiver mentioned above or both units.    -   The transmitter can further be equipped with additional signal        sources, which have such properties that they emit light into        the output that can be used for adjusting, synchronization or        information transmission within the quantum cryptographic        system.    -   The signal sources are light sources characterized in that the        light emitted by them and coupled into the quantum channel        either (a) has a well defined polarization orientation or (b)        has a relative phase position at different instants in time        or (c) has a relative phase position for different frequencies.        This is achieved by polarizers and/or phase elements which are        introduced into the light beam.    -   Light which has been coupled from different signal sources into        the quantum channel does not distinguish there in its        transversal beam profile.    -   Measuring units register the light, wherein (a) the polarization        of the light has a determined orientation, (b) the light coming        in at different instants in time has a determined relative phase        position, (c) the incoming light having different frequencies        has a determined relative phase position.    -   The receiver can be equipped with further measuring units for        the purpose of synchronization, adjusting, security checks and        information transmission within the quantum cryptography system.

The Figures show advantageous embodiments of the present invention. Thefeatures of the present invention which are believed to be novel are setforth with particularity in the appended claims. The invention, togetherwith further objects and advantages thereof, may best be understood byreference to the following description in conjunction with theaccompanying drawings.

FIG. 1 shows the preferred embodiment of a transmitter;

FIG. 2 shows a further preferred embodiment of a transmitter;

FIG. 2 a shows a version of the embodiment shown in FIG. 2 with reducedoverall length;

FIG. 3 shows a preferred embodiment of a receiver; and

FIG. 4 shows a further preferred embodiment of a receiver.

DETAILED DESCRIPTION Embodiment

Transmitter:

For simplifying existing quantum cryptography systems, the light of aplurality of signal sources is superimposed (FIG. 1). According to knownquantum cryptography systems, an external source (random generator,computer) determines by number which of a plurality of signal sources(11-14) emits a short light impulse. The signal sources used in thepresent embodiment are laser diodes which emit light with a well definedlinear polarization (polarization degree >97%). The laser diodes areoriented in a way that the polarization of the light emitted by themeach has a distortion of 45° with regard to that emitted by thepreceding diode. This means, depending on the number given by theexternal source, a light impulse having a polarization direction ofeither 0° (vertical polarization direction) (11), 45° (12), 90°(horizontal) (13) or 135° (14) is emitted. The laser diodes are arrangedin a semi-circle in such manner that similar parts of the emitted lightsuperimpose at the output due to the beam divergence (see the indicatedlight cone in the front view). For reducing the overall length or foradapting the beam divergence, a concave lens (convex mirror) can beintroduced in the beam path. The light is coupled into a monomode fiber(15) (quantum channel).

In an alternative embodiment (FIG. 2), the light emitted by laser diodes(21-24) having different orientations is coupled (optionally via a lensor a mirror) into a blind (25) serving for space filtering. The lightexiting here can be prepared via further lenses (26) (and optionallyblinds, telescope arrangement) as quantum channel for an opticaldirectional link.

In both embodiments, if desired, additional laser diodes, the emittedlight of which can be used for synchronization and adjusting, can bearranged in the center of the 4 laser diodes.

For reducing the overall length in a preferred embodiment, a conicallyshaped mirror element (27) is used (FIG. 2 a). This is arranged in thecenter of a circular arrangement of the laser diodes (28) in such amanner that the divergence of the light beams can be adaptedadvantageously to the divergence of the space filter (29) and thequantum channel (30).

Receiver:

In the receiver shown in FIG. 3, light comes via the monomode fiber(quantum channel) (31) from the transmitter and is collimated by a lens(32) or focused onto the detectors. Into the thus broadened light beam,a mirror (33) is partially introduced, which then reflects acorresponding part of the light to an analysis unit. The other part ofthe light passes a wave plate (34) and then reaches a further analysisunit. The wave plate is oriented in such a manner that linear polarizedlight having an orientation of 45° (−45°) is transformed into vertically(horizontally) polarized light. Both analysis units comprise apolarizing beam splitter (35, 36) and 2 single photon detectors (37-40).The polarizing beam splitter reflects vertically polarized light andtransmits horizontally polarized light. The signal of the single photondetectors mark the detected polarization: 0° (37), 45° (39), 90° (38),135° (40). The signal is prepared by a suitable electronic circuitry fora signal processing according to known quantum cryptography protocols.

In an alternative embodiment (FIG. 4), the light passes a space filterwith lenses (41), blinds (42). The light exiting the last blind isfocused onto the detectors by a lens (43). A plane parallel glass plate(44) is introduced into the right part of the light beam. It is tiltedin such a manner that the passing light is horizontally deferred to theright. Into the left partial beam, a wave plate (45) is introduced. Itis oriented in such a manner that linear polarized light having anorientation of 45°(−45°) is transformed into vertically (horizontally)polarized light. Both partial beams enter into a polarizing beamsplitter (46) which transmits horizontally polarized light and reflectsvertically polarized light. In this arrangement, both measuring channelsuse the same polarizing beam splitter. The transmitted or reflectedlight is registered by 4 single photon detectors (47-50). Thesedetectors are arranged in pairs in such a manner in the transmitted beamand in the reflected beam behind a mirror (51) that each left detectordetects light which has passed the wave plate before and each rightdetector detects light which has passed the plane parallel plate before.Here too, the signal of the single photon detectors mark the detectedpolarization and is prepared by a suitable electronic circuitry for asignal processing according to known quantum cryptography protocols.

The novelty of this invention is the use of the wave frontsuperimposition (in the transmitter) or the use of the wave frontsplitting (in the receiver). In contrast to conventional realizationswhich base on the principle of the superimposition or splitting by abeam splitter (amplitude superimposition or -splitting), the structurecan be simplified and components can be avoided which cause disturbingside effects. Thus, an improvement of the signal/noise behavior and ofthe stability as well as a reduction of adjusting and maintenancerequirements can be achieved. A miniaturization of the structure isfacilitated considerably. Due to the adjustable partition in thereceiver other protocols of the quantum cryptography [2] can be usedselectively without the replacement of existing components.

Fields of Application and Development

Quantum cryptography is the only method for key distribution whichguarantees a quantifiable security. Such a security is not available ifconventional software methods are used and although additionalapparatuses are required, there is a clear advantage over the keytransmission by couriers.

Due to the development of multi-functional information technologies, thenumber and the value of transmitted information increases rapidly, sothat a secure communication has an increasing importance for oursociety. This method is of economic interest for security-criticalapplications especially in the field of finance for banks andinsurances.

Specific embodiments of an apparatus and method for quantum cryptographyaccording to the present invention have been described for the purposeof illustrating the manner in which the invention may be made and used.It should be understood that implementation of other variations andmodifications of the invention and its various aspects will be apparentto those skilled in the art, and that the invention is not limited bythe specific embodiments described. It is therefore contemplated tocover by the present invention any and all modifications, variations, orequivalents that fall within the true spirit and scope of the basicunderlying principles disclosed and claimed herein.

1. A quantum cryptography system for a secure key generation comprising:one or more transmitters with an electronic transmitter circuit and atransmitter optic; one or more receivers with an electronic receivercircuit and a receiver optic; a quantum channel, which connects thetransmitter(s) with the receiver(s) and serves for the transmission oflight signals; at least 2 signal sources in the transmitter optic, whichemit light signals in distinguishable, also non-orthogonal, quantummechanical states; an electronic transmitter circuit, which allows onlyone of the signal sources to emit at one instant in time; and at least 2analysis channels in the receiver optic, which can analyzedistinguishable quantum mechanical conditions and forwarddistinguishable signals which identify the conditions to the electronicreceiver circuit; where the signal sources are spatially arranged insuch a manner that the wave fronts of the light signals emitted by themsuperimpose each other at least partially at the input of the quantumchannel; and/or the analysis channels are spatially arranged in such amanner that the wave fronts of the light signals coming from the quantumchannel are spatially split up and the quantum mechanical state of atleast two of the parts can be analyzed.
 2. The quantum cryptographysystem according to claim 1 where wave front changing optical componentscause the superimposing and/or the splitting of the wave fronts.
 3. Thequantum cryptography system according to claim 1 where laser diodes areused as signal sources.
 4. The quantum cryptography system according toclaim 1 where sources of single photons are used as signal sources. 5.The quantum cryptography system according to claim 1 where a quantummechanical state is given by the polarization of the light signals. 6.The quantum cryptography system according to claim 1 where spatiallyoriented signal sources and/or polarizers adjust the polarization of thelight signals in the transmitter optic.
 7. The quantum cryptographysystem according to claim 1 where polarization rotators change thepolarization of the light signals.
 8. The quantum cryptography systemaccording to claim 1 where spatially oriented polarizers and/ordetectors in the receiver optic analyze the polarization of the lightsignals.
 9. The quantum cryptography system according to claim 1 wherethe analysis channel comprises a single photon detector and/or apolarization rotator and/or an oriented polarizer, where differentanalysis channels can commonly use any polarization rotators and/orpolarizers present.
 10. The quantum cryptography system according toclaim 1 where a quantum mechanical state is given by the phasedifference between two or more components having an offset in time ofthe light signal.
 11. The quantum cryptography system according to claim1 where a non-adjusted interferometer in the transmitter optic generatesthe components having an offset in time of the light signals and anon-adjusted interferometer in the receiver optic processes the incominglight signals.
 12. The quantum cryptography system according to claim11, where the non-adjusted interferometer comprises a polarizing beamsplitter, and where a polarization caused by the signal sources,polarizers and/or polarization rotators results in a phase difference ofthe compounds having an offset in time of a light signal.
 13. Thequantum cryptography system according to claim 1 where signal sources inthe transmitter optic and the detectors around the receiver optic arearranged spatially so that an activated signal source/an activateddetector corresponds to a determined emitted/analyzed quantum mechanicalcondition.
 14. The quantum cryptography system according to claim 1where a quantum mechanical state is given by the phase differencebetween 2 or more frequency components of a light signal.
 15. A methodfor the secure key generation by means of quantum cryptographycomprising: providing one or more transmitters with an electronictransmitter circuit and a transmitter optic; providing one or morereceivers with an electronic receiver circuit and a receiver optic;providing a quantum channel, which connects the transmitter(s) with thereceiver(s) and serves for the transmission of light signals; emittinglight signals in distinguishable quantum mechanical states by at leasttwo signal sources in the transmitter optic; causing the electronictransmitter circuit to emit only one signal source at one instant intime; analyzing the distinguishable quantum mechanical conditions by atleast two analysis channels in the receiver optic and forwarding thedistinguishable signals identifying the conditions to the electronicreceiver circuit; where the signal sources are arranged in such a mannerthat the wave fronts of the light signals emitted by them superimpose atleast partially at the input of the quantum channel, and/or the analysischannels are arranged in such away that the wave front of the lightsignals coming from the quantum channel are spatially split up and thequantum mechanical state of at least two of the parts can be analyzed ina quantum mechanical condition.